Gravity study of the Djibouti area

Tectonophysics, 27 (1975) 177-185 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

GRAVITY STUDY OF THE DJIBOUTI AREA

J. MAKRIS*,

J. ZIMMERMANN*,

A. BALAN**

and A. LEBRAS**

* Institut fiir Geophysik der Universitiit Hamburg, Hamburg (Germany) **Bureau Gravimktrique international et Laboratoire de Tectonophysique de I ~~n~L~e~it~Paris VI, Paris (France) (Submitted

February 12, 1974; revised version accepted January 21, 1975)

ABSTRACT Makris, J., Zimmermann, J., Balan, A. and LeBras, A., 1975. Gravity study of the Djibouti area. Tectonophysics, 27: 177-185. In March and April 1972, 380 gravity stations were established in the T.F.A.I. The data were reduced to Bouguer anomalies of 5 mGa1 isolines. Qualitative and quantitative interpretation based on the gravity map, on seismic data and on the magnetics and physiography of the area revealed that: The crust of the Gulf of Tadjura and the central part of the T.F.A.I. is strongly oceanized and is the direct continuation of the Sheba Ridge. To the north of the Gulf, at the Dankali Mountains, the crust increases in thickness and most probably contains sialic fragments, indicating the continuation of the Danakil Chains in the T. F. A. I. To the south, the structure is bordered by a continental block, the Aysha Horst. The oceanization is concentrated only in the area of deep injections, marked by gravity maxima, whereas the rest of the area is to be described as sub-continental, The crust is attenuated from south (Aysha Horst) to north (Red Sea), from 30 to 15 km in thickness. The pattern of the gravity anomalies shows clearly liniations only along the coasts of the Gulf of Tadjura and the Straits of Bab el Mandeb. Inland the field breaks up into relative minima and maxima, indicating the fragmentation of the crust and the ‘triple junction’ nature of the area. The tectonic process is that of extension, with normal faults having maximum displacements at the northern border of the Gulf of Tadjura. The uppermost mantle has low velocity and density values due to thermal processes in the expanding zone. The state of the upper-mantle material must be that of partial melting due to. high temperatures of the order of 800” to 1,000“ C at about 15 km depth.

INTRODUCTION

The evolution of Afar as a ‘triple junction’ of the Red Sea, the East African Rift, and the Gulf of Aden can only be understood if the available geophysical and geological information on these areas is continuous. The fact that the territory of T.F.A.I. (see Fig. 1) had not been gravimetrically

4,??

LEGEND [73 <-4000[ft] m

LO09 -

‘j/jj!l!!ll’ 2000

n-2000

--IO00

~-1000

-+lOOO

a

-3.

;

c

-‘ 0

+lOOO - +2000

;2‘

Fig. 1. Topographic features of T.F.A.I.

and location of 2-D crustal model.

surveyed was a serious handicap in constructing a model for the development of Afar as a ‘triple junction’. The “Laboratoire de Tectonophysique” of the university of Paris and the “Institut fur Geophysik” of the University of Hamburg surveyed the area in March and April 1972, establishing 380 gravity stations. Their distribution corresponds to one station for every 60 square km. The aims of this study are to examine the tectonic lineations and to locate

179

areas of deep injections; to obtain the limits of the Danakil Alps and the Horst of Aysha, if any; to establish the relationship of the T.F.A.I. structures to those of the Gulf of Aden and the Afar; and to correlate the physical parameters obtained by other geophysical methods with those of the gravity field. TECHNICAL DETAILS AND ACCURACY

OF THE SURVEY

Two gravity parties were used for the field operations, each being equipped with the following instruments, viz., a gravity meter (one LaCosteRomberg, Type G No. 260 and one Worden-Master No. 859); a set of three altimeters Type Thommen 3B-4; and one aspiration hygrometer.

Fig. 2A. Gravity base connection Addis Ababa-Dire Units in mGal; see also Makris et al. (1973).

Dawa-Djibouti.

Entrance I

II

IYemenI

Fig. 2B. Djibouti Airport. Addis Ababa (K) Dire Dawa (Airport) Djibouti (J) Djibouti (K)

Potsdam system g = 977 481.48 g= 977 841.72 g = 978 249.84 g = 978 249.84

Reference code numbers: BGIf06813 J. BGI 06813 K.

Latitude = 11” 33’22”N. Longitude = 43” 09’20”E. Elevation = 9.28 m.

IGSN 71 977 466.91 977 827.91 978 235.27 978 235.27

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At every station the gravity field, the elevation differences, and the dry and moisture saturated air temperatures were measured. Gravity values were connected to the Djibouti Airport gravity station, point J (g = 978249.84 mGal) and were measured with an accuracy of kO.1 mGal (Fig. 2). Elevation differences between the gravity points were computed according to Jordan’s formula as modified by Moller (1962). Absolute station altitudes were obtained by connecting the elevation differences to the levelling net of the T.F.A.I. or to sea level. Stand corrections were also applied using the pressure-variation recordings of the “Bureau Meteorologique” at Djibouti. The mean accuracy of the altitudes, as obtained, is of the order of +2 to 5 m for flat terrain and ?5 to 10 m for the mountains. These accuracies do not influence the contour lines of the maps since the maximum errors they give are of the order of +2 mGal for the Bouguer anomalies and the isolines extrapolated have 5 mGa1 spacing. THE BOUGUER

MAPS

The gravity data were reduced to mean sea level and the mass reductions were spherical from Hayford zone A-02 (o-166.7 km) using a constant density of 2.67 g/cm3. The international gravity formula of 1930 was used to compute the theoretical gravity field. The Bouguer anomalies were extrapolated in 5 mGal isolines (Fig. 3). A higher resolution of the field can be achieved by greatly increasing the number of field points and the accuracy of their altitudes, since the observed gradients are very small. Along the coast of the Gulf of Aden the field has a linear pattern parallel to the seashore and its intensity decreases from the coast inland from +15 to -60 mGal. From N to S the isolines are deformed on entering the Gulf of Tadjura, marking a gravity high and indicating the existence of surplus masses below. Farther south the field recovers on entering the opposite coast at Djibouti and obtains its original distribution at the border of the Republic of Somali. Two minima, of -60 and -65 mGal, mark the areas of maximum elevation, one being situated to the north of the Tadjura Gulf at the Dankali Mountains and the other south of the Gulf at the Aysha Horst. At the western part of the area towards Afar, the field shows no clear lineations. Between Ghoubet Kharab and the Lake Abbe there is a gravity maximum of -20 mGal, which is surrounded by a series of minima. The fact that no predominant lineations exist indicates that we are dealing with the intersection area of a ‘triple junction’ of radial symmetry. CRUSTAL

MODELS

BASED

ON GRAVITY

AND SEISMIC DATA

Preliminary results of a refraction seismic study of the T.F.A.I. were published by Lepine et al. in 1972. Ruegg et al. (1973) discussed these results and their tectonic implications for the development of the T.F.A.I.

181

650

Fig. 3. Bouguer formula:

where

g YO 6gF

&gB

b

map of T.F.A.I.

43"

The anomalies

were computed

according

to the

= the measured gravity gravity reduction = 0.3083 (hs -h,) with h, = altitude of the gravity point, hc = reduction level = 0 m = Bouguer reduction, spherical upto Hayford zone 02, with p = 2.67 g/cm” = terrain reduction spherical to Hayford zone 02, with p = 2.67 g/cm3 = normal = free-air

and of Afar, northeast Ethiopia. We used the published velocity-depth distributions u(z) and by means of the Nafe-Drake curve (Talwani et al., 1959), estimated the density-depth function p(z). Two different crustal models were computed for a 400 km long N-S profile, extending from the

182

Red Sea near Assab to the Aysha Horst in Ethiopia (see Fig. 1). Seismic discontinuities are indicated in the models with heavy lines and areas of gradual velocity increase are marked with stripes. The northernmost seismically controlled area was extrapolated from the seismic refractional data given by Berckhemer et al. (1975), along the Assab-Assaita profile, see also Makris et al. (1975). The gradual velocity increase has been approximated gravimetrically with first-order discontinuities limiting intervals of mean velocity and density. Also the apparent discrepancy between the u(z)- and the p(z)-distribution used for the 2-D model computations at location 2 (Fig. 4), is due to fact that the seismic profile is located more to the east along to the Red Sea coast and not at the gravity minimum, which is crossed by the gravity model. In both models the crust south of the Gulf of Tadjura is gradually increasing in thickness to a standard continental at the southeastern Plateau in Ethiopia. To the north the crust becomes attenuated and has thickness values of approx. 15 km along the Red Sea coast and at the Straights of Bab el Mandeb. The main difference in the two models is in the structure of the Gulf of Tadjura, where no seismic control exists. In the first case the lateral density variation required by the gravity field is achieved by raising the 1eveI of the density layers without disrupting them. In the second model the 1.FA.I.

N-S

PROFILE

al -:ooo+--

1

-100

200

100

0

DISTANCE

IN

----------f-1000

300

KILOMETERS

_.

_” 3 35 __--

EJOd -H-Topography,

Fig. 4. Simplified

crustal

.

/ 60

.+ . Anomaly

model

Ohs.,

of T.F.A.I.

-

Anomoiy

computed

Colt

from gravity

and seismic

data.

183 T.FA.1.

N-S

PROFILE

50

~~~“:“.‘.‘-::1I--:-i: 0

-so

-100

IA-1000

-1000

0

-100

100 DISTANCE

-H-

Topography,

we.. AnomalyObt,

Fig. 6. 2-D model of the T.F.A.I.

200

-150

300

IN KILOMETERS

-

Anomaly Calc.

computed from gravity and seismic data.

crustal structure at the Gulf of Tadjura is disrupted and low velocity and density upper-mantle material is intruded within the lighter crust. The local gravity maximum situated in the Gulf is thus satisfied. Both models are mathemati~~ly possible and in agreement with the seismic constraints. Which of them is the correct one could only be decided upon by further seismic experiments within the Gulf of Tadjura along its axis. We believe, however, that due to the geologically proved extent of the Gulf, the second model, or an intermediate state between the first and the second model, is the better approximation. Also the existence of linear magnetic patterns (Whitmarsh, 1969) within the Gulf, indicates that the Gulf-structure has most probably developed due to the sea~oor-spreading mechanism and therefore model two is in better agreement with the mag netic data. Since rates of crustal spreading are presently being measured geodetically by the Institute de Physique du Globe de Paris (Ruegg et al., 19’73), it is hoped that their results will soon provide more constraints for our models. The densities used in the upper mantle are lower than normal, since the seismic results in the T.F.A.I. and also those in Afar (Berckhemer et al., 1975), revealed lower P-wave velocities than normal. Unfo~unately, the seismic penetration is not sufficient to delineate the u(z)-behaviour of the

184

P-wave velocities in the uppermost mantle. have been reported from the T.F.A.I., it is tical and lateral extend of the low-velocity uppermost part of the upper mantle, from

Since no travel-time anomalies not reliable to estimate the verand -density body, located in the gravity data only.

CONCLUSIONS

Gravity measurements reduced to Bouguer anomalies have revealed that in the T.F.A.I. two gravity minima of -. 60 and - 65 mGa1 are located in the regions of maximum elevation to the north and to the south of the Gulf of Tadjura. The Gulf itself is a region of increased gravity which is caused by material intruded from the upper mantle. Along the Gulf the crust is in a state of extension and the linear magnetic patterns indicate possible seafloorspreading processes presently active. Inland the gravity field shows no clear lineations and the distribution of the relative maxima and minima between Ghoubet Kharab and Lake Abbe is indicative for the triple-junction nature of this area. Along the coast the gravity field is linearly arranged and parallel to the morphological patterns. The combined interpretation of gravity and seismic data showed a continental type of crust at the southernmost part, the area of which attenuates gradually to the north. The crust is most probably disrupted at the Gulf of Tadjura and floored by mobilised upper-mantle material which is involved in seafloor-spreading processes, creating new oceanic crust. Fragmented sialic blocks build a great part of the crust of the T.F.A.I. and for this reason one can speak of oceanization due to spreading and attenuation but not of an oceanic crust of the Icelandic type (Makris, 1975). The physical state of the uppermost mantle shows significant changes, which are indicative of energy transport in the form of heat. The lower P-wave velocity than normal and its gradual increase can be interpreted by partially melted material. Also the relatively low level of the gravity field requires considerable mass deficiency in the upper mantle, even if we are unable to present models of its structure and dimensions. This requirement for low-density material in the upper mantle is also in agreement with the magnetotelluric measurements of Berktold (1974) who showed that in Afar the resistivity is 10-50 firn at about 15 km depth. This was interpreted by Berktold as corresponding to temperatures of approx. 800- 1000” C at this level. ACKNOWLEDGEMENTS

The authors are indebted to field operations, to the “Centre of T.F.A.I. conducted by Prof. franco-ethiopienne de chemins Funds for this program were

the T.F.A.I. authorities for supporting the d’Etudes Geologiques et de Developpement” Clin and Pouchan; and to the “Compagnie de fer”. provided by the “Bureau Gravimetrique

185

International”, the “Deutsche Forschungsgemeinschaft”, the “Ministere Frarqais des Arm&es”, the “Universite de Paris VI”. We thank Dr. S. Coron, of the B.G.I., and Dr. A. Guillaume, of the Tectonophysical Laboratory, for their help and their personal interest in this study. Prof. H. Menzel supported us in various ways and we wish to express our thanks to him. We are also indebted to Dr. P. Mohr for critical reading of this manuscript. Computations of the mass reductions and the final eompilation of the data were performed at the Rechenzentrum of the University of Hamburg. Mrs. M. Eilers put much effort into the production of drawings and maps.

REFERENCES Balan, A. and LeBras, A., 1972. Mesures gravimetriques en territoire franc;ais des Afars et des Issas. Rapport pour l’obtention du D.E.A. de Geophysique Appliquee, Univ. Paris VI, 1 vol. roneot., 55 pp. Berckhemer, H., Baier, B., Bartelsen, H., Behle, A., Burkhard, H., Gebrande, H., Makris, J., Menzel, H., Miller, H. and Vees, R., 1975. Deep seismic soundings in the Afar region and on the highland of Ethiopia. In: A. Pilger (Editor), Afar-Symposium. Stuttgart, 22 pp. Berktold, A., 1974. Ergebnisse der Magnetotellurik-Messungen in Athiopien. Herausgeber: A. Berktold, Miinchen. Girdler, R.W., 1970. An aeromagnetic survey of the junction of the Red Sea, Gulf of Aden, and Ethiopian rifts (a preliminary report). Philos. Trans. R. Sot. Lond., Ser. A, 267 : 359--368. Laughton, A.S., ~itmarsh, R.B. and Jones, M.T., 1970. The evolution of the Gulf of Aden. Philos. Trans. R, Sot. London, Ser. A, 267: 227-266. Lepine, J.C., Ruegg, J.C. and Steinmetz, L., 1972. Seismic profiles in the Djibouti area. In: R.W. Girdler (Editor), East African Rifts. Tectonophysics, 15: 59-64. Makris, J., 1975. Afar and Iceland - A geophysical comparison. In: A. Pilger (Editor), Afar-Symposium. Stuttgart, 22 pp. Makris, J., Zimmermann, J., Bachem, H.C. and Ritter, B., 1973. Gravity survey of South Afar, Ethiopia. Geophys., 39: 280-290. Makris, J., Menzel, H., Zimmermann, J. and Gouin, P., 1975. Gravity field and erustal structure of North Ethiopia. In: A. Pilger (Editor), Afar-Symposium. Stuttgart, 22 pp. Mijller, D., 1962. Beitrgge zur barometrischen Hijhenmessung. Verijffehtlichung der Deutschen Geodlitischen Kommission, Reihe C, Heft 52, 89 pp. Roberts, D.G. and Whitmarsh, R.B., 1969. A bathymetric and magnetic survey of the Gulf of Tadjura, western Gulf of Aden. Earth Planet. Sci. Lett., 5: 253-258. Ruegg, I.C. and Lepine, I.C., 1973. Aperc;u sur quelques etudes g~ophysiques effectuees en Afar. Rev. Geogr. Phys. Geol. Dynam. (2), XV (4): 415-424. Talwani, M., Sutton, G.H. and Worzel, J.L., 1959. A crustal section across the Puerto Rico Trench. J. Geophys. Res., 64: 1545-1555. Whitmarsh, R.B., 1969. Magnetic anomaly chart of the Gulf of Aden. Philos. Trans. R. Sot. London, Ser. A, 267, Wallet, No. 1181.